The present invention generally relates to an olefin polymerization system and process. More particularly, the present invention relates to an olefin polymerization system and process that include an improved way of removing heavies from diluent and/or unreacted monomer withdrawn from and recycled to the polymerization reactor.
Olefin polymerization processes may be conducted under slurry conditions. Such polymerization processes may be carried out in a loop reactor in which monomers are polymerized to form a slurry of solid polymer particles in a liquid medium. Portions of the slurry are withdrawn from the loop reactor through take-offs devices such as settling legs and continuous take-offs.
The slurry is then processed to separate the liquid medium (for example, a liquid diluent) from the solid polymer particles. Flashline heaters and flash chambers have been used to vaporize the liquid diluent through a combination of temperature and pressure adjustments, thereby separating the diluent from the solid polymer particles. Additionally, other types of equipment, such as purge columns, separating drums and cyclones, have been used for separation and other downstream treatment.
Efficient slurry polymerization processes typically recycle as much diluent as they reasonably can. That is, as the diluent is vaporized, diluent is processed for recycling back to the loop reactor. As the liquid diluent is vaporized at various stages of the polymerization process after the slurry is withdrawn from the loop reactor, various compounds and contaminants may also be vaporized along with the diluent.
xe2x80x9cHeaviesxe2x80x9d are liquid components heavier than the diluent, and are often vaporized along with the diluent. During the recycling process, a vapor stream containing the diluent and heavies may be condensed and returned to the loop reactor as a recycle stream of the diluent and heavies. Heavies may concentrate or accumulate within the loop reactor and decrease reactor efficiency and/or cause damage within the loop reactor. For example, heavies buildup within the reactor may degrade product quality and/or distort reactor control making reactor problems and blockage more likely.
Another recycling technique separates a slip stream comprising diluent and minor amounts of monomer from the bulk of the flashed fluid components. The slip stream is subjected to olefin removal to give an essentially olefin-free stream for recycle to a catalyst preparation area. The slip stream may be taken off before and/or after a diluent recycle cooler. The slip stream(s) is then passed to a heavies removal column. The heavies removal column removes heavies from the slip stream, which can then be used to prepare a catalyst mud or slurry. The bulk of monomer-containing recycle diluent is passed to a diluent recycle surge (storage) vessel, and a recycle stream is passed back to the reactor. If desired, other purification steps such as water removal can be carried out on this recycle stream.
An efficient system and process of recycling diluent in a slurry polymerization process are desired. Further, an improved heavies removal system and process, which remove heavies from the diluent to be recycled to the reactor, are desired.
As one aspect of the present invention, a process for recycling a liquid medium withdrawn from a slurry polymerization reactor is provided. The method comprises separating a first vapor stream from a slurry comprising a liquid medium and solid polymer particles, wherein the vapor stream comprises at least the medium and heavies. A first portion of the first vapor stream is passed to a first condensing zone, and a first portion of the first vapor stream is condensed to form a first liquid. A second portion of the first vapor stream is passed to a collection zone without substantial condensation. A second liquid and a second vapor stream are separated in the collection zone. The second liquid is passed to a heavies purification zone, and the medium is recycled from the second vapor stream without fractionating to remove heavies.
As another aspect of the present invention, a recovery and purification system for a liquid medium from a slurry polymerization is provided. The system comprises a first fluid passage connected at one end to a slurry polymerization reactor and connected at an opposite end to a flash gas separator, for separating a vapor stream comprising the medium from solid polymer particles. The system also comprises a vapor removal line connected at a top portion of the flash gas separator, for transporting the vapor stream from the flash gas separator. The vapor removal line is connected to a first condenser and also to a vapor bypass line that provides a bypass around the condenser. A bypass valve controls the flow of vapor through the vapor bypass line. The output of the condenser and the opposite end of the bypass line are both fluidly connected to a liquid collection tank. A liquid delivery line is at a bottom portion of the liquid collection tank, and a vapor recycle line is at a top portion of the liquid collection tank. A second condenser is fluidly connected to the vapor recycle line, and a second fluid passage connects the output of the second condenser to the slurry polymerization reactor.
The present invention may include control apparatus or steps associated with the bypass valve. For example, the bypass valve can be in informational communication with at least one of a temperature controller downstream of the liquid collection tank and a level controller positioned within the liquid collection tank. The bypass line is advantageous because it is difficult to operate a condenser to produce only a small amount of liquids. The present invention is able to produce a relatively small amount of liquids because the first vapor stream (which is the flash gas from the first flash chamber) is split, with a first portion passing through a condenser and a second portion bypassing the condenser. The flash gas passing through the condenser may be entirely or nearly entirely condensed into liquid, or a small part (for example, about 1-10%) may be condensed, or some other proportions.
Downstream of the condenser, the hot flash gas is combined with the cold liquid and mixed and allowed to come to an equilibrium (at least with respect to temperature). After this mixing, the liquid that remains is more concentrated in the heavier components. The desired amount of liquid is set by the flow controller on the liquid line. The level in the tank is controlled by adjusting the temperature controller, which in turn adjusts the bypass valve so that a sufficient amount of vapor bypasses the condenser to generate enough liquid to satisfy the flow controller. To accomplish the desired amount of heavies removal, from about 0.1 to about 20% of the flash gas alternatively from about 0.5% to about 10%, alternatively from about 1% to about 5%, will be converted into liquids by the condenser. At times it may be desirable to collect no liquids. At other times (for example, during resin transitions), it may be desirable to maximize liquids generation and collect more than usual.
The present invention may also include a heavies column or other heavies removal treatment at an opposite end of the heavies delivery line. A flow controller and/or a pump can be disposed along the heavies delivery line.
A static mixer can be fluidly connected to and downstream from the first condenser, and the static mixer is upstream from and fluidly connected to the liquid collection tank. The static mixer is employed to quicken the formation of an equilibrium mixture after the hot flash gas and cold liquid are combined. It is desirable to have equilibrium conditions before entering the collection tank, at least a temperature equilibrium. A recycle tank may be disposed along the diluent recycle line downstream of the flash gas condenser.